Conjugated Polyelectrolytes as Water Processable ... - ACS Publications

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Conjugated Polyelectrolytes as Water Processable Precursors to Aqueous Compatible Redox Active Polymers for Diverse Applications: Electrochromism, Charge Storage, and Biocompatible Organic Electronics James F. Ponder, Jr., Anna M. Ö sterholm, and John R. Reynolds* School of Chemistry and Biochemistry, School of Materials Science and Engineering, Center for Organic Photonics and Electronics, Georgia Tech Polymer Network, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States S Supporting Information *

ABSTRACT: An organic soluble precursor polymer was prepared by direct (hetero)arylation polymerization of 3,4ethylenedioxythiophene (EDOT) with a solubilizing, 3,4-propylenedioxythiophene (ProDOT) derivative bearing esterfunctionalized side chains. Chemical defunctionalization of the polymer, using base to hydrolyze the esters, yields a conjugated polyelectrolyte (CPE) that is readily soluble in water. This aqueous soluble CPE can then be processed using high-throughput coating methods from water-based inks. Postprocessing functionalization of the polymer film using dilute acid creates a solvent resistant film that is compatible with both organic and aqueous electrolyte systems for redox switching. The introduction of an unfunctionalized EDOT unit results in a soluble polymer that has a low oxidation potential and that is highly electroactive and pseudocapacitive in a wide voltage range (2 V in propylene carbonate-based electrolytes and 1.55 V in aqueous electrolytes) making it an attractive material for lightweight and flexible supercapacitors. Films of this copolymer demonstrate exceptionally rapid redox switching (10 V/s) and higher mass capacitance in aqueous electrolyte solutions than in organic solutions. Supercapacitors incorporating the solvent resistant copolymer exhibit symmetric charge/discharge behavior at currents of up to 20 A/g (1 s discharge) and are able to maintain >75% of the initial capacitance over 175 000 cycles using 0.5 M NaCl/water as the device electrolyte. Rapid electrochromic switching (∼0.2 s) from vibrant blue to colorless is also maintained in this salt− water electrolyte. The versatility of this polymer is further shown in a series of organic and aqueous electrolyte systems, including biologically compatible electrolytes (NaCl/water, Ringer’s solution, and human serum) and even sport drinks (Gatorade and Powerade), demonstrating the robustness of this polymer to differing ionic conditions. Based on these results, it is apparent that this polymer and similar systems have great potential in multiple electrochemical applications such as electrochromic devices, supercapacitors, and biocompatible devices.

1. INTRODUCTION

but not soluble in aqueous electrolytes) have seen slower progress. Two such areas are biologically compatible organic electrochemical transistors (OECTs) for neural signaling and redox controlled drug release. Both of these applications require redox active polymers that can be reversibly doped and dedoped in aqueous media. Similarly, ECDs and SCs incorporating

Electroactive polymers have become increasingly important in the field of organic electronics, especially with the emergence of organic bioelectronics that makes use of the ability of these polymers to transport both electrons and ions.1,2 While many applications, such as electrochromic devices (ECDs)3,4 and supercapacitors (SCs),5,6 have advanced significantly over the past decade as a result of extensive material and device development, other applications, especially those that rely on aqueous compatible materials (i.e., a material that is redox active © XXXX American Chemical Society

Received: February 24, 2017 Revised: April 14, 2017

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DOI: 10.1021/acs.chemmater.7b00808 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials

Scheme 1. General Overview of the Conversion of an Organic Soluble Polymer (OS-Polymer) to the Water-Soluble CPE Form, Followed by Acid Treatment to a Solvent Resistant (SR-Polymer) and Aqueous/Organic Compatible Film along with Images and Contact Angle Measurements of the OS-Polymer and SR-Polymer Discussed in This Article

while useful and worth continued investigation, are not a universal solution for obtaining aqueous compatibility and, furthermore, do not necessarily provide aqueous solubility. The limited solubility in organic solvents that is provided by these side chains also reduces the number of possible repeat unit structures that can be used, which in turn limits our ability to tune redox and color properties. An approach to induce water solubility is to synthesize conjugated polyelectrolytes (CPEs) that have ionic functionalities (such as sulfonates) tethered to the backbone of, e.g., thiophene, 3,4-ethylenedioxythiophene (EDOT), and pyrrolebased polymers.8−11 However, the main issue with CPEs is that films of these polymers can redissolve in aqueous electrolytes, limiting their aqueous electrolyte compatibility. One way to circumvent dissolution is to immobilize the active polymer with a redox inactive cross-linker, such as 3-glycidyloxypropyl trimethoxysilane (GOPS), to render the film insoluble in water.12 While it is an effective approach to stabilize PEDOT:PSS or other CPEs in aqueous electrolytes, the addition of, e.g., GOPS does influence the redox properties of the CP,12 most likely as a result of the lower ionic mobility in cross-linked films. In this report, we adapt a method13,14 for the preparation of a conjugated polymer with ester-based side chains that has high solubility in common organic solvents resulting in a material that is easily purified and fully characterized using standard methods (NMR, GPC, etc.) and then hydrolyzed to allow for aqueous processing. This is followed by a mild acid wash to render the polymer films solvent resistant (SR) and allow for redox switching in both organic and aqueous electrolyte solutions. While this method was previously used for aqueous processing and switching of nonoptimized electrochromic (EC) polymers, here we explore the scope and versatility of this method for a broader range of applications requiring redox activity. As a key synthon, a 3,4-propylenedioxythiophene (ProDOT) monomer was synthesized bearing multiple alkyl ester-based side chains having the carbonyl side of the ester closest to the propylene bridge. Using this design, upon hydrolysis, the carboxylate that is formed is covalently attached to the polymer backbone.13 The ester-functionalized monomer is brominated

nonflammable and nontoxic aqueous electrolytes would be safer for research and development as well as disposable consumer products. These applications will also benefit from the increased ionic mobility and conductance (as shown in Table S1) of aqueous electrolytes (relative to organic electrolytes) as it relates directly to how fast these devices switch color and charge/ discharge. Additionally, processing the redox active films from water has considerable advantages, particularly the lack of toxic fumes. Over the past decade the field of conjugated polymers (CPs) has been moving away from electropolymerization as a deposition method to soluble polymers that can be characterized with a broader range of techniques (e.g., NMR, GPC, viscometry, etc.) and processed using a variety of roll-to-roll compatible methods. Solubility in CPs is typically achieved by functionalizing the polymer backbone with long, nonpolar hydrocarbonbased (alkyl) chains that provide high solubility in organic solvents due to increased conformational entropy and, as a result, ease their isolation and purification. To achieve aqueous compatibility, the surface polarity of the polymer film must be modified to reduce the hydrophobic nature imparted to the film by the alkyl side chains. One embodiment of this concept is the incorporation of glycol-based side chains onto a conjugated backbone that, while not typically providing aqueous solubility, are sufficiently hydrophilic to allow for redox switching in aqueous media. This was recently demonstrated in a study by Nielsen et al., where organic soluble triethylene glycol functionalized polymers were found to be effective in OECTs, with accumulation mode devices (i.e., a device that is OFF at zero gate voltage) yielding higher transconductance values, the main figure of merit for OECTs, than poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) depletion mode devices (i.e., a device that is ON at zero gate voltage).7 In spite of these excellent results, the above-mentioned polymer family demonstrates the poor organic and aqueous solubility often seen in CPs with glycol-based side chains, with only two of the five polymers prepared being sufficiently soluble for gel permeation chromatography (GPC) molecular weight estimation. From this example we can see that glycol-functionalized polymers, B


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Chemistry of Materials and then polymerized with a heteroaryl dihydrogen species (specifically EDOT) using direct (hetero)arylation polymerization (DHAP) conditions to yield an organic soluble (OS) polymer that can be dissolved in common solvents including tetrahydrofuran, toluene, and chloroform. Following purification of the OS-polymer, typical characterizations were performed to confirm the repeat unit structure and estimate molecular weight. As previously mentioned, the side chains that impart organic solubility and processablility are normally hydrocarbon-based and therefore hydrophobic and do not provide aqueous solubility. This is illustrated in Scheme 1 (top left), where the OS-polymer selectively dissolves in chloroform and films of this polymer have relatively high contact angle (91.7 ± 1.0°) with water. The hydrophobic character prevents hydrated electrolyte ions from penetrating into the film upon redox cycling thereby inhibiting counterion insertion and electrochemical doping of the conjugated polymer (CP) in aqueous media.15 In order to overcome this hydrophobicity, the ester side chains were hydrolyzed to the corresponding CPE, and the polymeric carboxylate salt was found to dissolve in water (up to 10 mg/mL) due to the strong ion−dipole interactions between the carboxylates and water. As illustrated in Scheme 1 (top right), the water-soluble (WS) polymer can be cast using various methods to form films that can be further protonated via a dilute acid wash. The resulting film, now bearing carboxylic acid side chains, is insoluble in both organic and aqueous media and is termed solvent resistant (SR). The polar carboxylic acids on the surface, and in the bulk, of the film results in a hydrophilic surface with a low contact angle (29.2 ± 4.1°) that does not repel solvated ions and is capable of redox switching in both organic and aqueous media. However, if processing from organic solvents is required but aqueous compatibility is desired, the OS-polymer can be hydrolyzed in the solid state to produce the ionic polymer film and then acid treated to obtain the SR version.14 Here, we demonstrate the versatility of this material and method by evaluating the SR-polymer in various aqueousbased electrolytes and electrochemical devices.

Figure 1. Synthetic route from monomers to the final solvent resistant polymer film. Post-Polymerization Hydrolysis To Form Water-Soluble PE (WS-PE). The OS-PE (400 mg) and a stir bar were added to a 100 mL round-bottom flask with 50 mL of 2.0 M KOH/methanol solution. The solution was degassed by bubbling argon through it for 30 min. The round-bottom flask was equipped with a reflux condenser and covered in argon and heated to reflux (65 °C) for 24 h while vigorously stirring. After cooling to room temperature, the suspension was filtered over a 0.45 μm filter and washed with methanol, chloroform, and diethyl ether. The solid polymer cake was dried under high vacuum overnight to yield 246 mg (83.4% recovery) of a water-soluble CPE, WS-PE. Film Preparation/Post-Processing Functionalization. The WSPE was dissolved in deionized water and either spray cast (4 mg/mL) onto ITO-glass substrates on a hot plate (∼60 °C) or drop cast (2 mg/ mL, 2 μL, 4 μg of polymer) onto glassy carbon button electrodes and airdried for 30 min. The films were then dipped into a 1 M solution of ptoluenesulfonic acid (pTSA)/methanol (or 1 M hydrochloric acid (HCl)/methanol) for 10 min (open to air). The resulting solvent resistant films were washed with methanol and allowed to air-dry. All film measurements (except for EC characterizations) were made in triplicate to ensure reproducibility. Type I Supercapacitor Device Fabrication. To evaluate this polymer for supercapacitor applications, electrodes were prepared by drop casting films onto glassy carbon electrodes (area: 0.07 cm2). Both of the electrodes were electrochemically conditioned by cycling them in the respective electrolytes (25 cycles for aqueous electrolytes and up to 100 cycles for lithium bis(trifluoromethanesulfonyl)imide (LiBTI)/ propylene carbonate (PC)). One of the polymer-coated electrodes was then electrochemically reduced to convert the polymer film to the charge neutral form (−1.0 V vs Ag/Ag+ for LiBTI/PC and −0.8 V vs Ag/ AgCl for aqueous electrolytes) for 30 s, and the other polymer films were converted to the fully oxidized form (1.0 V vs Ag/Ag+ for LiBTI/PC and 0.8 V vs Ag/AgCl for aqueous electrolytes) for 30 s. The two polymercoated electrodes were assembled into a Swagelok-type setup and separated by a cellulose-based separator soaked in electrolyte. Before device assembly, the electrolyte solutions were degassed via argon bubbling, but no additional measures were taken to remove oxygen and moisture from the device as they were built on the bench in an ambient environment. All device measurements (except galvanic cycling and lifetime tests) were made on at least three devices to ensure reproducibility.

2. EXPERIMENTAL SECTION Materials and Methods. Additional figures, NMR, materials, and instrumentation details can be found in the Supporting Information. Organic Soluble Poly(ProDOT-alt-EDOT) (OS-PE) Synthesis. The dibromo-tetraester ProDOT monomer (monomer 1, Figure 1) (1.00 g, 0.869 mmol, 1.00 equiv) and freshly distilled EDOT (monomer 2, Figure 1) (0.123 g, 0.869 mmol, 1.00 equiv) were added to a 38 mL pressure vessel equipped with a stir bar. Pd(OAc)2 (4 mg, 0.02 equiv), pivalic acid (26 mg, 0.3 equiv), K2CO3 (0.299 g, 2.5 equiv), and DMAc (8.7 mL, 0.2 M, degassed with argon) were added, and the vessel was sealed under argon and placed in an oil bath at 140 °C and stirred overnight (∼14 h). The reaction was cooled to ambient temperature, the reaction mixture was precipitated into methanol, and the resulting solution/precipitate was filtered into a Soxhlet thimble. The polymer was purified via Soxhlet extraction using methanol, acetone, and hexanes and then dissolved in chloroform. Approximately 40 mg of a palladium scavenger (diethylammonium diethyldithiocarbamate) and ∼40 mg of 18-crown-6 were added to the polymer/chloroform solution which was concentrated under vacuum and then stirred for 2 h at 40 °C and subsequently precipitated into methanol. After filtering, washing with methanol, and drying the precipitate under vacuum, 0.928 g (94.4%) of a dark purple solid was obtained. 1H NMR (800 MHz, CD2Cl4, 50 °C) δ8.24 (t, 2H), 7.8 (d, 4H), 4.4 (d, 4H), 4.3−3.8 (br, 8H), 1.7 (s, 4H), 1.45−1.0 (br, 36H), 1.0−0.6 (br, 29H). Anal. Calcd for C63H86O14S2 C 66.88, H 7.66, S 5.67, Found C 66.85, H 7.62, S 5.73. Mn = 25.4 kDa, Đ = 2.8 (THF GPC at 35 °C vs polystyrene standards)

3. RESULTS AND DISCUSSION Polymer Synthesis. In this study, a poly(ProDOT-altEDOT) (PE) repeat unit structure was chosen as we have previously found structures of this basic composition to be suitable for both EC and charge storage applications.16,17 The OS form of the polymer (OS-PE) was prepared via the DHAP C


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here that the backbone and degree of polymerization are identical for the two polymers as one is a precursor of the other. As expected from a polymer containing a substantial fraction of EDOT moieties in the repeat unit, the OS-PE has a low onset of oxidation determined by differential pulse voltammetry (DPV) to be −0.55 V relative to Fc/Fc+ (shown in Figure S2a).16 This is approximately the same onset of oxidation measured for the previously reported16,19 polymers with the same conjugated backbone, but bearing 2-ethylhexyl (−0.54 V vs Fc/Fc+) or 2hexyldecyl (−0.48 V vs Fc/Fc+) side chains (as shown in Figure S2b). The SR form, bearing the shortest side chains (film cast from water, then treated with 1 M pTSA/methanol), exhibits similar peak current but has an onset of oxidation that is 0.21 V lower (−0.76 V vs Fc/Fc+) than the OS form. This lower onset of oxidation results in the SR polymer having a broader electroactive window than not only the OS form but also the previously reported 2-ethylhexyl or 2-hexyldecyl functionalized analogues of the same polymer, indicating that the role of side chains on redox property tuning may be more intricate than generally thought. The lower oxidation potential of SR-PE is likely the result of a decrease in the steric bulk that occurs when we cleave the large ethylhexyl chains and replace them with protons. We do not observe a change in the oxidation potential of OS-PE that has been treated with pTSA/methanol, ruling out the possibility that the lower oxidation potential is purely due to solvent annealing or acid doping effects. The fact that the SR form can maintain a high current density throughout a larger electroactive window directly translates into a higher film capacitance, a property that is important not only for charge storage applications but also biosensors and OECTs. Redox Behavior of the Solvent Resistant Polymer in Aqueous Electrolytes. As a result of the hydrophilic nature of the SR-PE, it is also highly electroactive in aqueous electrolytes of neutral or low pH, as shown in Figure 3a for both LiBTI/H2O and salt water (NaCl/H2O). Comparing the SR-PE in PC and water (Figure 2 and 3a) with LiBTI serving as the electrolyte salt, we see minimal differences in the redox response of the films at low scan rates (≤250 mV/s), aside from the potential window being reduced to accommodate the stable window of water.20 NaCl is an equally effective electrolyte salt for this polymer (Figure 3a), and no degradation is observed over repeated cycling. Monitoring the peak current as a function of scan rate allows us to assess the kinetic limitations of the redox cycling in both PC and aqueous electrolyte systems. SR-PE films in both

(Figure 1) of EDOT and a functionalized dibromo-ProDOT monomer bearing ester side chains using standard conditions previously reported.18 The polymer was purified using Soxhlet extraction, with the polymer dissolving in the chloroform fraction in excellent yield (94.4%) with a number-average molecular weight of 25 kDa (Đ = 2.8). The structure was confirmed via elemental analysis and high-resolution NMR (as shown in Figure S1). A portion of this polymer was hydrolyzed by refluxing the solid in a solution of KOH in methanol to convert it to the CPE form. As the methanol insoluble OS-PE is hydrolyzed, the solid polymer disperses into a fine powder as the side chains are removed. After filtering, washing, and drying, the polymer was found to selectively dissolve in water and to be insoluble in chloroform, as shown in Scheme 1. Comparing Redox Behavior of Organic Soluble and Solvent Resistant Polymer Films in Organic Electrolytes. As shown in Figure 2, both the OS and the SR form of the

Figure 2. Cyclic voltammograms of drop cast films of the OS-polymer (purple line) and the SR-polymer (red line) on glassy carbon electrodes in 0.5 M LiBTI/PC at a scan rate of 50 mV/s.

polymer are electroactive in propylene carbonate (PC)-based electrolytes, but, more importantly, this figure also demonstrates that the redox behavior is not solely determined by the electroactive backbone but also by the side chains as we observe large differences in the cyclic voltammograms (CVs) that are directly related to the side chain cleavage. It is important to recall

Figure 3. (a) Cyclic voltammograms recorded at 50 mV/s and (b) peak currents (with standard deviation) as a function of scan rate of films of SR-PE in various electrolytes (0.5 M). D


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PE in their respective stable potential windows without compromising the peak current, film capacitance (Figure S6a and S6b), or electrochemical reversibility. This is of particular importance as most OECT measurements are typically performed in NaCl/H2O, which, while an excellent first test, is not comparable to actual biological electrolyte systems.7,15,23 Because of this success, we decided to extend the family of electrolytes to commercial sports drinks to determine the versatility and robustness of SR-PE. Gatorade and Powerade were both found to be effective electrolytes for SR-PE in their stable potential windows (Figure 4). This is an important result as the question of long-term redox stability is a constant concern in the field of CPs. This redox activity and stability in biological electrolytes demonstrates the potential for this polymer, and others of the same kind, in bioelectronics and other applications requiring complex or “harsh” electrolytes. Specifically, the combination of high film capacitance and aqueous compatibility is necessary for OECT devices as the figure of merit, transconductance, is directly proportional to the capacitance of the redox active film (see Equation S1).7,21 Supercapacitors Incorporating Solvent Resistant Polymers. To show the potential of these SR films in an electrochemical device, we prepared symmetrical Type I supercapacitors5 (schematic of the device architecture is shown in Figure S7) that incorporated either the OS-PE or the SR-PE as the active pseudocapacitive material on both electrodes with LiBTI/PC (OS-PE and SR-PE), LiBTI/H2O (SR-PE only), and NaCl/H2O (SR-PE only) as the device electrolytes, as shown in Figure 5. Devices incorporating the OS-PE have a linear scan rate dependence below 250 mV/s (tdischarge: 3.2 s). At higher scan rates, the mass capacitance drops rapidly and only ca. 10% of the initial capacitance is retained at 10 V/s (Figure S8). As we have shown for the films in a three-electrode cell in Figure 3b, the current becomes diffusion limited at scan rates above 250 mV/s, so this result was not unexpected. In contrast, devices incorporating the SR-PE maintain their current and mass capacitance up to a charge/discharge rate of 1 V/s (tdischarge: 0.8 s), suggesting that the reduced steric bulk and higher degree of polarity of the side chains enhance the electrolyte−polymer interactions. Because of the lower electrolyte conductance (3 mS/cm, as shown in Table S1) of LiBTI/PC compared to the aqueous analogues (LiBTI/H2O = 21 mS/cm, NaCl/H2O = 46 mS/cm), devices incorporating LiBTI/PC are not able to retain their capacitance above 1 V/s whereas the aqueous devices are able to retain ∼80% of their original capacitance even at 10 V/s (tdischarge: 0.08 s), as seen in Figure 5b. We have not been able to determine the underlying reason for the large deviation in mass capacitance of the LiBTI/PC devices in Figure 5b. As shown in Figure 5c, the fill factor (a measure of deviation from ideal behavior17,24) of devices incorporating NaCl/H2O remains unchanged from 50 mV/s up to 10 V/s. Devices incorporating LiBTI/H2O only exhibit a decrease of ∼15% of their fill factor at 10 V/s, while LiBTI/PC devices decreased to approximately half their initial values. Previous literature has demonstrated that hydrated ions are more mobile in polar environments, such as CPEs or in polymer films with polar side chains, e.g., glycols, alcohols, or acids rather than alkyl based side chains.15,25−28 Our results clearly support these conclusions, as we observe a large difference in performance between the OS and SR-PE films, and suggest that the alkyl side chains are limiting ion movement into/ out of the film. To more closely mimic actual device operation, the devices were evaluated by galvanic cycling where the charge/discharge

aqueous electrolytes exhibit a linear scan rate dependence, as seen in Figure 3b, with peak current values within error of each other up to 10 V/s. This is important as most polymers, including the SR-PE and same backbone with branched alkyl side chains (ethylhexyl or hexyldecyl, see Figure S3), begin to show diffusion-controlled behavior at scan rates above 250 mV/s in PC-based electrolytes. Using NaCl/H2O electrolyte resulted in a smaller sample-to-sample deviation in peak current than the LiBTI/H2O electrolyte despite all films being prepared identically. The film mass capacitance values (as determined by cyclic voltammetry at 50 mV/s) (Figure S4) are comparable in both LiBTI/H2O (42 ± 6 F/g) and NaCl/H2O (33 ± 4 F/g). While it is difficult to make direct comparisons with literature as capacitance values are highly dependent on whether they are determined by cyclic voltammetry or electrochemical impedance spectroscopy (EIS) and on the scan rate, if determined by the former, the mass capacitance of SR-PE is comparable to PEDOT:PSS, which has a reported mass capacitance of 39 F/ g21 (determined by EIS and assuming a density of 1 g/cm3). Electropolymerized PEDOT has a higher mass capacitance of 92 F/g,22 not only due to the absence of solubilizing chains or a matrix polymer, but also because it unlike PEDOT:PSS, is able to completely depdope and achieve full depth of discharge. It is important to note that the final protonation step that converts the CPE to a SR polymer does not require the use of pTSA but can be done using any acidic solution sufficiently strong to protonate the carboxylate groups, i.e., using an acid that has a pKa that is lower than 4. To demonstrate this, we performed the protonation step in a solution of 1 M HCl in methanol which was found to be equally effective for rendering the films solvent resistant without changing the nature of the redox response, as seen in the CV and DPV traces of SR-PE in LiBTI/H2O shown in Figure S5. Biologically Compatible and Novel Electrolytes. As a result of the effectiveness of NaCl/H2O as an electrolyte, the scope of the aqueous compatibility was further explored. First, Ringer’s solution was tested, as it is compositionally similar to biological fluids, such as cerebral spinal fluid, and is often used in medical research. Second, human serum from blood was tested. From the CVs in Figure 4, it can be seen that both of these biologically relevant media are as effective electrolytes for the SR-

Figure 4. Cyclic voltammograms of the SR-PE in biologically relevant electrolytes (Ringer’s solution and human serum) and sports drinks (Powerade and Gatorade) at a scan rate of 50 mV/s. E


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under ambient conditions, was monitored over 175 000 charge/ discharge cycles at 1 V/S (Figure 6b), and these devices are able to maintain >75% of their initial capacitance during these tests. This clearly demonstrates that water does not degrade these dioxythiophene-based CP over time, and we expect the observed drop in current could be reduced by device preparation under inert atmosphere to remove oxygen from the device. The next step for this research is to extend this method to other materials with repeat units optimized for charge storage and electrodes appropriate for a commercial device. In the end, you cannot achieve a much more cost-effective or environmentally benign solution than salt water. Toward High Contrast and Rapidly Switching Salt Water-Based Electrochromic Materials. To demonstrate yet another application for this water processable/solvent resistant polymer system, films were spray cast onto ITO-coated glass from water and evaluated as electrochromic materials. As shown in Figure 7a (the corresponding transmittance spectrum can be found in Figure S9), this material is effective as a high contrast EC material with a vibrant blue neutral state (λmax = 613 nm) and a highly transmissive oxidized state. Similarly to other ProDOTxEDOTy copolymers,16 the polaronic/radical cation absorbance is found at ∼1000 nm, and the bipolaron/dication band >1500 nm exhibits minimal tailing into the visible, resulting in the highly color neutral oxidized state. Films coated to a transmittance of only 2% at λmax in the colored state can maintain a high contrast (ΔT @ λmax = 68.0%) and switch to an oxidized state with a transmittance of ∼70%. The rapid redox switching seen in the supercapacitors incorporating this polymer translates to a rapid change in color as shown in the chronoabsorptiometry in Figure 7b (and magnified in Figure S10) where 95% of a full contrast switch is achieved in just 0.17 s for a SR-PE film deposited to a transmittance of ∼7% in the neutral state. This is significantly faster than what is typically seen for EC polymers (including the same backbone structure with branched alkyl side chains in organic electrolytes) that require 0.5 to >2s to complete a full switch between the two states. This is not only due to the high ionic conductance of the salt water as this specific polymer is switching faster than previously reported aqueous compatible electrochromic polymers 14,29,30 but is also due to the incorporation of EDOT units into the backbone, which we have previously shown16,19 results in rapidly switchable EC polymers.

4. CONCLUSIONS AND PERSPECTIVES In summary, this reported methodology of forming solvent resistant films from water-soluble polymers that are processed as aqueous-based inks has allowed for redox switching in a large variety of organic, inorganic, and biologically compatible electrolytes. Supercapacitor using the solvent resistant side chain have a higher mass capacitance and are able to maintain this capacitance at significantly higher scan/discharge rates than the corresponding polymer with hydrophobic side chains. NaCl in water was found to offer the best device performance with highly reversible behavior at scan rates of up to 10 V/s and charge/ discharge currents of 20 A/g. The use of salt water did not compromise stability, as the devices continued to operate effectively even after 175 000 charge/discharge cycles. The significance of this cannot be overstated, as the long-term stability of conjugated polymer-based redox active devices (such charge storing supercapacitors and ECDs) is vital for commercial use.

Figure 5. (a) Representative CVs of Type I supercapacitors incorporating SR films and various electrolytes (with dashed lines indicating ideal performance), (b) the mass capacitance, and (c) the fill factor of these devices as a functions of scan rate.

behavior is monitored as a function of current density. As shown in Figure 6a for devices incorporating NaCl/H2O as device electrolyte, the charge/discharge behavior is highly symmetrical, as expected in an ideal supercapacitor, up to exceptionally high current densities of 20 A/g, corresponding to a ∼1.1 s discharge time. This demonstrates that these devices have low internal resistance, fast ion diffusion, and fast charge transfer kinetics. The cycling stability of devices incorporating salt water and assembled F


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Figure 6. (a) Galvanic cycling of a Type I SR-PE supercapacitor using a 0.5 M NaCl/H2O electrolyte and (b) select CVs of an identical device over 175 000 charge/discharge cycles at 1 V/s.

Figure 7. (a) Spectra recorded in 50 mV increments from −0.80 to 0.70 V (vs Ag/AgCl) and photos taken at −0.80 and 0.70 V. (b) Transmittance at λmax as a function of switching time of a spray-cast film of the SR-PE on ITO glass in 0.5 M NaCl/H2O switching from the charge neutral state (−0.80 V vs Ag/AgCl) to the oxidized state (0.70 V vs Ag/AgCl).

With this information in mind we should again consider bioelectronics. The combination of aqueous compatibility, high capacitance, excellent EC properties, and stability in aqueous media over extended cycling suggest that this material, and others like it, would be useful for bioelectronic applications, specifically OECTs. The high electrochromic contrast is potentially useful is this area as well, as monitoring color changes facilitates the determination of the extent of doping throughout the bulk of a film and, correspondingly, ion uptake and swelling of the polymer in various electrolyte systems.7,26,31,32

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ORCID

John R. Reynolds: 0000-0002-7417-4869 Author Contributions

J. F P. performed the synthesis, characterization, and device work. A. M. Ö . consulted on device preparation and performed data analysis. All authors contributed to the writing of this manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge Mr. Augustus W. Lang for his assistance in contact angle measurements and Dr. Michel de Keersmaecker for performing galvanic cycling measurements. This work was funded by the Office of Naval Research (Grant: N00014-14-1-0399) for developing aqueous compatible capacitive materials and by the National Science Foundation (Grant: CHE-1506046) for the improvement of aqueous switching electrochromic polymers.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b00808. Additional procedures, characterizations, materials, and instrumentation details (PDF)

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AUTHOR INFORMATION

Corresponding Author

REFERENCES

(1) Rivnay, J.; Owens, R. M.; Malliaras, G. G. The Rise of Organic Bioelectronics. Chem. Mater. 2014, 26, 679−685.

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Chemistry of Materials (2) Rivnay, J.; Inal, S.; Collins, B. A.; Sessolo, M.; Stavrinidou, E.; Strakosas, X.; Tassone, C.; Delongchamp, D. M.; Malliaras, G. G. Structural control of mixed ionic and electronic transport in conducting polymers. Nat. Commun. 2016, 7, 11287. (3) Neo, W. T.; Ye, Q.; Chua, S.-J.; Xu, J. Conjugated polymer-based electrochromics: Materials, device fabrication and application prospects. J. Mater. Chem. C 2016, 4, 7364−7376. (4) Cai, G.; Wang, J.; Lee, P. S. Next-Generation Multifunctional Electrochromic Devices. Acc. Chem. Res. 2016, 49, 1469−1476. (5) Bryan, A. M.; Santino, L. M.; Lu, Y.; Acharya, S.; D’Arcy, J. M. Conducting Polymers for Pseudocapacitive Energy Storage. Chem. Mater. 2016, 28, 5989−5998. (6) Schon, T. B.; McAllister, B. T.; Li, P.-F.; Seferos, D. S. The rise of organic electrode materials for energy storage. Chem. Soc. Rev. 2016, 45, 6345−6404. (7) Nielsen, C. B.; Giovannitti, A.; Sbircea, D.-T.; Bandiello, E.; Niazi, M. R.; Hanifi, D. A.; Sessolo, M.; Amassian, A.; Malliaras, G. G.; Rivnay, J.; McCulloch, I. Molecular Design of Semiconducting Polymers for High-Performance Organic Electrochemical Transistors. J. Am. Chem. Soc. 2016, 138, 10252−10259. (8) Patil, A. O.; Ikenoue, Y.; Wudl, F.; Heeger, A. J. Water soluble conducting polymers. J. Am. Chem. Soc. 1987, 109, 1858−1859. (9) Krishnamoorthy, K.; Kanungo, M.; Ambade, A. V.; Contractor, A. Q.; Kumar, A. Electrochemically polymerized electroactive poly(3,4ethylenedioxythiophene) containing covalently bound dopant ions: poly{2-(3-sodiumsulfinopropyl)-2,3-dihydrothieno[3,4-b][1,4]dioxin}. Synth. Met. 2001, 125, 441−444. (10) Zeglio, E.; Vagin, M.; Musumeci, C.; Ajjan, F. N.; Gabrielsson, R.; Trinh, X. T.; Son, N. T.; Maziz, A.; Solin, N.; Inganäs, O. Conjugated Polyelectrolyte Blends for Electrochromic and Electrochemical Transistor Devices. Chem. Mater. 2015, 27, 6385−6393. (11) Reynolds, J. R.; Sundaresan, N. S.; Pomerantz, M.; Basak, S.; Baker, C. K. Self-doped conducting copolymers: a charge and mass transport study of poly{pyrrole-CO[3-(pyrrol-1-YL)propanesulfonate]}. J. Electroanal. Chem. Interfacial Electrochem. 1988, 250, 355−371. (12) Håkansson, A.; Han, S.; Wang, S.; Lu, J.; Braun, S.; Fahlman, M.; Berggren, M.; Crispin, X.; Fabiano, S. Effect of (3-glycidyloxypropyl)trimethoxysilane (GOPS) on the electrical properties of PEDOT:PSS films. J. Polym. Sci., Part B: Polym. Phys. 2017, 55, 814−820. (13) Shi, P.; Amb, C. M.; Dyer, A. L.; Reynolds, J. R. Fast Switching Water Processable Electrochromic Polymers. ACS Appl. Mater. Interfaces 2012, 4, 6512−6521. (14) Amb, C. M.; Beaujuge, P. M.; Reynolds, J. R. Spray-Processable Blue-to-Highly Transmissive Switching Polymer Electrochromes via the Donor−Acceptor Approach. Adv. Mater. 2010, 22, 724−728. (15) Giovannitti, A.; Sbircea, D.-T.; Inal, S.; Nielsen, C. B.; Bandiello, E.; Hanifi, D. A.; Sessolo, M.; Malliaras, G. G.; McCulloch, I.; Rivnay, J. Controlling the mode of operation of organic transistors through sidechain engineering. Proc. Natl. Acad. Sci. U. S. A. 2016, 113, 12017− 12022. (16) Ponder, J. F.; Ö sterholm, A. M.; Reynolds, J. R. Designing a Soluble PEDOT Analogue without Surfactants or Dispersants. Macromolecules 2016, 49, 2106−2111. (17) Ö sterholm, A. M.; Ponder, J. F.; Kerszulis, J. A.; Reynolds, J. R. Solution Processed PEDOT Analogues in Electrochemical Supercapacitors. ACS Appl. Mater. Interfaces 2016, 8, 13492−13498. (18) Estrada, L. A.; Deininger, J. J.; Kamenov, G. D.; Reynolds, J. R. Direct (Hetero)arylation Polymerization: An Effective Route to 3,4Propylenedioxythiophene-Based Polymers with Low Residual Metal Content. ACS Macro Lett. 2013, 2, 869−873. (19) Kerszulis, J. A.; Johnson, K. E.; Kuepfert, M.; Khoshabo, D.; Dyer, A. L.; Reynolds, J. R. Tuning the painter’s palette: Subtle steric effects on spectra and colour in conjugated electrochromic polymers. J. Mater. Chem. C 2015, 3, 3211−3218. (20) Sigg, L. Redox Potential Measurements in Natural Waters: Significance, Concepts, and Problems. Redox: Fundamentals, Processes, and Applications; Schüring, J., Schulz, H. D., Fischer, W. R., Böttcher, J.,

Duijnisveld, W. H. M., Eds.; Springer: Berlin, Heidelberg, 2000; pp 1− 12. (21) Rivnay, J.; Leleux, P.; Ferro, M.; Sessolo, M.; Williamson, A.; Koutsouras, D. A.; Khodagholy, D.; Ramuz, M.; Strakosas, X.; Owens, R. M.; Benar, C.; Badier, J.-M.; Bernard, C.; Malliaras, G. G. Highperformance transistors for bioelectronics through tuning of channel thickness. Sci. Adv. 2015, 1, 1−5. (22) Snook, G. A.; Chen, G. Z. The measurement of specific capacitances of conducting polymers using the quartz crystal microbalance. J. Electroanal. Chem. 2008, 612, 140−146. (23) Inal, S.; Rivnay, J.; Leleux, P.; Ferro, M.; Ramuz, M.; Brendel, J. C.; Schmidt, M. M.; Thelakkat, M.; Malliaras, G. G. A High Transconductance Accumulation Mode Electrochemical Transistor. Adv. Mater. 2014, 26, 7450−7455. (24) Ö sterholm, A. M.; Shen, D. E.; Dyer, A. L.; Reynolds, J. R. Optimization of PEDOT Films in Ionic Liquid Supercapacitors: Demonstration as a Power Source for Polymer Electrochromic Devices. ACS Appl. Mater. Interfaces 2013, 5, 13432−13440. (25) Simon, D. T.; Gabrielsson, E. O.; Tybrandt, K.; Berggren, M. Organic Bioelectronics: Bridging the Signaling Gap between Biology and Technology. Chem. Rev. 2016, 116, 13009−13041. (26) Stavrinidou, E.; Leleux, P.; Rajaona, H.; Khodagholy, D.; Rivnay, J.; Lindau, M.; Sanaur, S.; Malliaras, G. G. Direct Measurement of Ion Mobility in a Conducting Polymer. Adv. Mater. 2013, 25, 4488−4493. (27) Inal, S.; Rivnay, J.; Hofmann, A. I.; Uguz, I.; Mumtaz, M.; Katsigiannopoulos, D.; Brochon, C.; Cloutet, E.; Hadziioannou, G.; Malliaras, G. G. Organic electrochemical transistors based on PEDOT with different anionic polyelectrolyte dopants. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 147−151. (28) Pacheco-Moreno, C. M.; Schreck, M.; Scaccabarozzi, A. D.; Bourgun, P.; Wantz, G.; Stevens, M. M.; Dautel, O. J.; Stingelin, N. The Importance of Materials Design To Make Ions Flow: Toward Novel Materials Platforms for Bioelectronics Applications. Adv. Mater. 2017, 29, 1604446. (29) Reeves, B. D.; Unur, E.; Ananthakrishnan, N.; Reynolds, J. R. Defunctionalization of Ester-Substituted Electrochromic Dioxythiophene Polymers. Macromolecules 2007, 40, 5344−5352. (30) Beaujuge, P. M.; Amb, C. M.; Reynolds, J. R. A Side-Chain Defunctionalization Approach Yields a Polymer Electrochrome SprayProcessable from Water. Adv. Mater. 2010, 22, 5383−5387. (31) Hütter, P. C.; Fian, A.; Gatterer, K.; Stadlober, B. Efficiency of the Switching Process in Organic Electrochemical Transistors. ACS Appl. Mater. Interfaces 2016, 8, 14071−14076. (32) Mawad, D.; Artzy-Schnirman, A.; Tonkin, J.; Ramos, J.; Inal, S.; Mahat, M. M.; Darwish, N.; Zwi-Dantsis, L.; Malliaras, G. G.; Gooding, J. J.; Lauto, A.; Stevens, M. M. Electroconductive Hydrogel Based on Functional Poly(Ethylenedioxy Thiophene). Chem. Mater. 2016, 28, 6080−6088.

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